The present invention relates to semiconductor packages, and more particularly, to a ball grid array (BGA) semiconductor package having a heat spreader for effectively performing heat dissipation.
With rapid advances and unceasing improvements in the electric technology, electronic products such as cellular phone, notebook, and PDA need to be miniaturized in both weight and size without sacrificing functions. To fulfil the tendency for miniaturization of electronic products, the integrated circuit technology adopted to integrate and increase the density of electric circuit and electronic components on a semiconductor chip plays an important role, and so does the IC packaging technology.
Purposes of packaging or encapsulating a semiconductor chip are to protect the semiconductor chip from being affected by outer contaminants such as moisture and dust, to provide an electrical connection between the semiconductor chip and a printed circuit board (PCB), and to provide structural support for preventing the semiconductor chip from being deformed or damaged. A conventional semiconductor package having a leadframe for mounting a semiconductor chip, such as Quad Flat Package (QFP) or Thin Quad Flat Package (TQFP) has a limitation to increase the number of the I/O pins and to decrease the pitch between any two adjacent I/O pins. As a result, Ball Grid Array (BGA) type semiconductor packages are developed to solve the problems existing in conventional leadframe type semiconductor packages. The BGA semiconductor package typically has an array of solder balls attached to the substrate for bearing a semiconductor chip. As the solder balls are allowed to be disposed under the semiconductor chip, the BGA semiconductor package has an optimized pin count.
For the BGA semiconductor package, heat generated from a semiconductor chip in operation is dissipated to the outside of the package via the route comprised of the semiconductor chip, silver paste, conductive traces on a top surface of a substrate, the substrate, conductive traces on a bottom surface of the substrate, and solder balls. However, the thermal conductive path for a BGA semiconductor package is so long that the heat generated by the semiconductor chip can not be effectively and efficiently dissipated out of the package body via the aforementioned heat dissipating path, while the density of semiconductor components and electric circuit on the semiconductor chip is considerably increased. Thus, a BGA semiconductor package having a heat spreader for improving thermal conductivity has been disclosed in U.S. Pat. No. 5,736,785. As shown in FIGS. 10 and 11, the semiconductor package of U.S. Pat. No. 5,736,785 has a heat spreader 116, which sits on the top surface of the substrate 104 to which the die 102 is adhered, for effectively dissipating heat. There are first supporters 116f at corners of the square-shaped heat spreader 116 for preventing the die 102 from being moved and damaged while the heat spreader 116 is disposed in the package body 112. A shallow dish-like recessed portion 116a protrudes from the central portion of bottom surface of the heat spreader 116 to contact a top surface of the die 102 for effectively dissipating heat generated by the die 102 to the outside of a package body through the heat spreader 116. Meanwhile, four hemispherical downward projections 116c are provided on corners of the heat spreader 116. And the downset of the hemispherical downward projections 116c is adapted to be deeper than that of the shallow disk-like recessed portion 116a so that when mounted on the top surface of the substrate 104, the heat spreader 116 is supported by the hemispherical downward projections 116c without causing pressure to the die 102. There are four openings 116b formed on the heat spreader 116 for allowing the packaging resin to flow into space under the heat spreader 116 during molding process.
Such semiconductor package can effectively increase the efficiency of heat dissipation by making the heat spreader 116 in contact with the die 102. However, for packaging the die 12 and the heat spreader 116 with packaging resin, the packaging process needs to take extra steps to adhere the bottom surface of the shallow disk-like recessed portion 116a of the heat spreader 116 to the die 102, when compared with conventional packaging operation for BGA semiconductor packages. The packaging process for such semiconductor package comprises the following steps of; (a) attaching the die 102 to the top surface of the substrate 104, and making electrical connection from the die 102 to the substrate 104 by gold wires 108; (b) preparing a fixture for the substrate 104 to allow the die 102 to be received in an opening of the fixture; (c) daubing the die 102, a portion o fate substrate 104, and gold wires 108 with epoxy resin through the opening of the fixture; (d) setting the heat spreader 116 on the top surface of the chip 102 via the opening of the fixture, and adjusting a location of the heat spreader 116 by using the fixture to downwardly press the heat spreader 116 for letting the top surface of the heavy spreader 116 to be covered with the epoxy resin, and to attach the heat spreader 116 to the top surface of the chip 102 via the epoxy resin; (e) removing the fixture, and curing the epoxy resin; and (f) proceeding the molding process.
However, the aforementioned process, which takes time longer than conventional processes, has drawbacks as follows: (a) A fixture is needed to adjust the location of the heat spreader on the top surface of the chip 102 prior to adhering the heat spreader 116 to the chip 102. However, there is required a permissible gap between the edge of the heat spreader 116 and the fixture to allow the heat spreader 116 be clipped and adopted inside the fixture, so that the location of the heat spreader 116, corresponding to the chip 102, is not precisely set over the semiconductor chip 102, and then, an off-center phenomena resulted from the misalignment between the heat spreader 116 and the chip 102 would affect the outlook of the semiconductor package. (b) Compared with conventional process, although each step, from aforesaid step (b) to (e), of the molding process of U.S. Pat. No. 5,736,785 can be accomplished in the same work area, however, extra steps which are step (b) to step (c) are still needed for the molding process, and thus, the molding process is more complicated than convention one and the production cost would increase. (c) Due to different expansion coefficient of the heat spreader 116 and the chip 102, after the heat spreader 116 has been adhered to the top surface of the chip 102 and the molding process has been accomplished, a de-lamination phenomena resulted from a interface between the heat spreader 116 and the chip 102 would degrade qualities of products. (d) In addition, the heat spreader 116 should be adhered to the chip 102 and the chip 102 is sensitive to pressures inside the encapsulant so that when the upper and lower mold half are combined together for introducing the epoxy resin into cavities of the two mold halves, the pressure resulted from the resin flow inside cavities will exert to the chip 102 via the heat spreader 116, causing the chip 102 to be cracked; meanwhile, a thermal stress caused by high temperature effect during the molding process is still remained inside the heat spreader 116, and that would crack the chip 102, while the resin is cured. (e) Further, there are a dish-like recessed portion 116a of the central region of the heat spreader 116, and four hemispherical downward projections 116c on four corners, so that during the molding process, the epoxy resin which flows into a cavity between the upper mold half and the chip 102 would be retarded by the dish-like recessed portion 116a and the hemispherical downward projections 116c, and then, a turbulent resin flow phenomena immerges. Therefore, voids are formed inside the cured epoxy of the encapsulant, and degrade qualities of finished products.
Accordingly, an object of the present invention is to provide a semiconductor package having an exposed heat spreader on a top surface of a substrate, in which the placement of the heat spreader can be easily carried out without adjustment operation.
Another purpose of the present invention is to provide a semiconductor package having an exposed heat spreader, which can improve heat dissipation.
Still another object of the present invention is to provide a semiconductor package having an exposed heat spreader which is simple in assembly and low in production cost.
A further object of the present invention is to provide a semiconductor package having an exposed heat spreader which can increase the yield of the products.
Still a further object of the present invention is to provide a semiconductor package having an exposed heat spreader for decreasing voids inside the cured resin according to steady resin flow during the molding process.
To achieve above and other objects of the present invention, a semiconductor package, comprising a substrate having a first surface and a second surface; first conductive traces formed on the said first surface of the substrate; second conductive traces formed on the second surface of the substrate and electrically connected with the first conductive trace via a plurality of vias formed in the substrate; a semiconductor chip mounted on the first surface of the substrate and being electrically connected to the first conductive trace; a plurality of solder balls attached to the second surface of the substrate for electrical connection with the second conductive traces, a heat spreader disposed on the first surface of the substrate the heat spreader comprising a lower portion formed with an opening in the center for receiving the semiconductor chip portion, and a connecting portion for connecting the lower portion and the upper portion in a manner that the upper portion is raised to a height above the opening of the lower portion, wherein the lower portion is formed with a plurality of positioning members outwardly extending from edges of the lower portion for preventing the heat spreader on the substrate from being dislocated during molding process and, a plurality of supporting members to support the heat spreader over the substrate to allow a top surface of the upper portion to be exposed to the exterior of an encapsulant subsequently formed to encapsulate the semiconductor chip and a portion of the heat spreader, and to keep the semiconductor chip form being in contact with the heat spreader.
The predetermined distance between the lower portion of the heat spreader and the substrate is defined as the downset of the supporters, and large enough for making the top surface of the upper portion of the heat spreader be exposed to the outside of the cured encapsulant, and contact with the air, after the molding process.
The supporters are uniformly punched and formed on the lower portion. Each supporter has an indented portion which can be a dish-like recessed portion, a cone-shaped portion, or a hemispherical downward projection, or a vertical sectional view of supporters is V-shaped or U-shaped, without restrictions. The supporter also can be downward tongue-shaped by uniformly cutting the lower portion.
A top surface of the upper portion of the heat spreader is a plane-like shape, but, however, it can also be a wave-like shape, a square-wave-like shape, or a plurality of short cylinders extend from the top surface for increasing a surface area of the top surface to effectively dissipate heat during operation.
Besides, contacting portions are formed on each side, each corner, or two opposite sides, of the heat spreader.
After the molding process, the top surface of the upper portion of the heat spreader is allowed to be exposed to the outside of the encapsulant, without flash thereon by having the height of the heat spreader greater than that of a cavity of an encapsulating mold consisting of an upper mold half and a lower mold half. Therefore, while the upper and lower mold halves are combined together, the inner surface of the upper mold half would tightly contact with the upper portion of the heat spreader to slightly press the heat spreader, during the molding process. Due to the slight deformation of the heat spreader, which is caused by the downward pressure from the upper mold half to the heat spreader, the top surface of the upper portion of the heat spreader closely contacts with the inner surface of the upper mold half. Thus, the flash phenomena would not happen. Meanwhile, the difference between those two vertical distances is less than 0.2 mm, but is preferred to be less than 0.1 mm.